The objective of this continuing project is to investigate the mechanisms and pathway through which neurons in the rostral ventrolateral medulla (RVL) regulate the cerebral circulation. Excitation of neurons of the RVL elevates regional cerebral blood flow (rCBF) within the cerebral cortex without elevating cerebral metabolism. These neurons also mediate half of the cerebrovascular vasodilation to hypoxia. Three studies will be undertaken which combine methods of microelectrical and chemical stimulation, neuronal inactivation, recording of bioelectrical activity and rCBF, and tract tracing and immunocytochemistry in anesthetized rats. Project I will test the hypotheses that (a) excitation of a distinctive group of neurons in the caudal RVL (RVLc) elevates rCBF in the cerebral cortex; (b) these RVLc neurons are reflexively excited from FN, NTS, and oxygen sensitive neurons of rostral RVL (RVLr); (c) the pathway to the cortex from RVLc is indirect and relays through a subcortical projection probably localized to Zl; (d) vasodilatory effects of excitation of RVLc of Zl neurons depend upon the integrity of cortical neurons for the final expression of elevated rCBF; (e) the vasodilator pathway represented in the RVLs is the common pathway through which activation of FN, NTS, and RVLr also produces a slowing of the EEG and elevation of rCBF. The study will provide evidence for the first time of a full pathway of the brainstem linking visceral afferent activity with the circulation and electrical brain activity.